State of health determination for batteries is important for electric bikes, electric motorcycles, electric vehicles, electric forklifts, and energy storage systems that require a wide range and a dynamic battery energy as the power source. The absence of a proper mechanism in the determination of the state of health of the batteries may result in unnoticed shut down (or failure) of a battery system during service. In general, the unnoticed shut down (or failure) of the battery system is caused by one of the batteries (or battery sets) connected in series that possesses a smaller capacity or higher resistance. This problem is especially severe for lithium ion battery systems since an absolute cut-off voltage is usually set for each battery (or battery set) connected in series. One scenario that may lead to unnoticed shut down of the battery system is when one of the batteries (or battery sets) connected in series reaches a cut-off condition before the battery system's working lower-cut-off voltage is reached. In this case, a battery system shut down may result without notice.
On the other hand, the determination of state of health may not be easily conducted without complicated and prolonged processes. For example, probing individual battery or battery set capacity separately involves cycling each battery. This is a long and complicated process that cannot be conducted on a daily basis, and hence increases the risk of unnoticed shut down of the battery system during users' daily operation.